Noise suppression is used to suppress any background acoustic sound superimposed on the desired speech signal, while preserving the characteristics tics of the speech. In most applications, the noise suppressor is implemented as a pre-processor to the speech encoder. The noise suppressor may also be implemented as an integral part of the speech encoder.
There also exist implementations of noise suppression algorithms that are installed in the networks. The rationale for using these network-based implementations is that a noise reduction can be achieved also when the terminals do not contain any noise suppression. These algorithms operate on the PCM (Pulse Code Modulated) coded signal and are independent of the bit-rate of the speech-encoding algorithm. However, in a telephony system using low speech coding bit-rate (such as digital cellular systems), network based noise suppression can not be achieved without introducing a tandem encoding of the speech. For most current systems this is not a severe restriction, since the transmission in the core network usually is based on PCM coded speech, which means that the tandem coding already exists. However, for tandem free or transcoder free operation, a decoding and subsequent encoding of the speech has to be performed within the noise-suppressing device itself, thus breaking the otherwise tandem free operation. A drawback of this method is that tandem coding introduces a degradation of the speech, especially for speech encoded at low bit-rates.